Ruzsa’s genus-one problem

Constructed by exploiting the congruence 279 ≡ 104 ≡ 4 (mod 25). Started from the N=104 record witness [0,1,5,14,18,19,39,84], preserved its residue pattern modulo 25 by choosing suitable 25-lifts inside Z/279Z, then extended the lifted 8-set by three additional elements. The resulting 11-set is locally rigid under direct extension and small r→r+1 exchanges tested in the search.